Optical beam combiner

ABSTRACT

An optical beam combiner for combining a plurality of light beams comprises: a plurality of spherical mirrors; and a flat mirror, the plurality of spherical mirrors and the flat mirror configured to form at least one multiple pass light beam optical arrangement for receiving the plurality of light beams and for superimposing spot images of the light beams onto a single location with a single incident angle. In addition, a waveguide-based optical White cell comprises: a waveguide having front and rear edges, the inside surfaces thereof being coated with a reflective material, wherein the front edge including an input section for the passage of at least one light beam into the waveguide; at least one waveguide lens disposed in front of the inside surface of the rear edge to form a plurality of waveguide spherical mirrors at the rear edge; a plurality of angled micro mirrors disposed at the inside surface of the front edge; and the plurality of waveguide spherical mirrors and the coated front edge configured to form at least one waveguide White cell.

This utility application claims the benefit of the filing date of theU.S. Provisional Application 60/588,729, entitled “Optical BeamCombiner”, and filed Jul. 16, 2004.

BACKGROUND OF THE INVENTION

The present invention relates to optical devices, in general, and moreparticularly, to an optical beam combiner for receiving a plurality oflight beams and superimposing spot images of the plurality of lightbeams onto a single location with a single incident angle.

Generally, an optical cross-connection device, like a White cell opticalswitch, for example, comprises a plurality of optical elements disposedin a predetermined spatial three dimensional pattern for directing oneor more light beams from an input through a plurality of reflections toan output. Multiple light beams may bounce through various stages of thedevice simultaneously. A problem arises at the final or output stage ofthe White cell cross-connection device where the multiple light beamsare ultimately directed from different spatial locations and differentincidence angles. Thus, the multiple light beams will illuminate spotsin various locations within the region of the output stage. Accordingly,each light beam of the multiplicity has a distinct incidence angledepending onto which region of the output stage it is being directed.This variation in the angle of incidence complicates the coupling of thelight beams into an optical fiber or a light detector.

The present invention is intended to overcome or at least mitigate thisdrawback to the optical coupling in the output stages of opticalcross-connection devices.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an optical beamcombiner for combining a plurality of light beams comprises: a pluralityof spherical mirrors; and a flat mirror, the plurality of sphericalmirrors and the flat mirror configured to form at least one multiplepass light beam optical arrangement for receiving the plurality of lightbeams and for superimposing spot images of the light beams onto a singlelocation with a single incident angle.

In accordance with another aspect of the present invention, an opticalbeam combiner for combining an array of light beams comprises: aplurality of spherical mirrors; and a flat mirror, the plurality ofspherical mirrors and the flat mirror configured to form at least onemultiple pass light beam optical arrangement for receivingsimultaneously the array of light beams and for superimposing spotimages of each light beam of the array onto a single location with asingle incident angle.

In accordance with yet another aspect of the present invention, awaveguide-based optical White cell comprises: a waveguide having frontand rear edges, the inside surfaces thereof being coated with areflective material, wherein the front edge including an input sectionfor the passage of at least one light beam into the waveguide; at leastone waveguide lens disposed in front of the inside surface of the rearedge to form a plurality of waveguide spherical mirrors at the rearedge; a plurality of angled micro mirrors disposed at the inside surfaceof the front edge; and the plurality of waveguide spherical mirrors andthe coated front edge configured to form at least one waveguide Whitecell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an exemplary free space White cell opticalarrangement.

FIGS. 2 a, 2 b and 2 c are top view illustrations depicting examples ofoperation of the exemplary White cell optical arrangement.

FIGS. 3 a, 3 b and 3 c are front mirror illustrations depicting multiplepass light beam illuminations resulting from various operations of theexemplary White cell.

FIG. 4 is an illustration of an exemplary dual White cell opticalcross-connection device.

FIG. 4 a is a light beam connectivity diagram suitable for use indescribing the operations of the exemplary dual White cell opticaldevice.

FIG. 5 is an illustration depicting multiple pass light beamilluminations of the faces of the mirrors of a dual White cell deviceusing an array of micro mirrors as a common mirror element for bothWhite cells.

FIG. 6 is an illustration depicting an embodiment of a dual White celldevice using an array of micro mirrors as a common mirror element forboth White cells.

FIG. 6 a is an illustration of an output region of a White cell opticaldevice showing the illuminations from two different light beams.

FIG. 7 is an illustration depicting an alternate embodiment of a dualWhite cell device using an array of micro mirrors as a common mirrorelement for both White cells.

FIG. 8 is an illustration depicting an exemplary optical beam combinersuitable for embodying one aspect of the present invention.

FIG. 9 is an illustration of a mirror face of the optical beam combinershowing multiple pass light beam illuminations of the face thereof.

FIG. 10 is an illustration of the optical beam combiner showing themultiple pass light beam illuminations on the mirror face.

FIGS. 11 and 12 are front and isometric perspective views, respectively,of an exemplary waveguide-based White cell optical arrangement suitablefor embodying another aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An optical switch based on the principles of an optical White cell willexemplify an optical cross-connection device for the purposes ofdescribing one or more embodiments of the present invention. The opticalWhite cell is an example of a multi-pass light beam optical system forgenerating a series of spot illuminations in sequence for an input lightbeam as will be better understood from the following description. Otherexamples of multi-pass light beam systems include a Herriot cell or anyof the alternative spot pattern generators disclosed in U.S. Pat. No.6,266,176. For the present example, a White cell comprising a set ofthree spherical mirrors with identical radii of curvature will be used.The multi-pass system of spherical mirrors will refocus the beamcontinuously within the White cell. One of the White cell's sphericalmirrors may be replaced with an array of micro mirrors which may be madeusing micro-electromechanical systems (MEMS) techniques and willhereinafter be referred to as the MEMS micro mirrors, MEMS array or MEMSdevice.

Each of the micro mirrors of the MEMS device may be independently tiltedto different angles. Also, multiple light beams may be directed toreflect or bounce off of the optical elements within the White cellsimultaneously, and each light beam may be focused to illuminate a spoton a different micro mirror on each bounce or pass. Thus, in theexemplary optical switch of the present embodiment, there is anopportunity to switch a light beam with the MEMS device toward a newdestination on each bounce. In addition, the number of possibleattainable outputs of the exemplary switch will depend on the number ofbounces that the light beams make in the White Cell. So, the number ofattainable outputs may be controlled by controlling the number ofbounces.

This White cell technology offers a highly scaleable all-opticalcross-connect switch for a large number of ports (N inputs×N outputs),that avoids the effects of beam divergence and high precision anglecontrol of the MEMS micro mirrors. Because several beams may bounceinside the White cell, each one of them may be controlled individuallyin such a way to control the destination of each beam. That is, eachbeam can be directed to any of multiple output regions. As noted above,however, on the final stage each beam will have a distinct incidenceangle depending on which output region a particular beam is directed,which complicates the coupling into a fiber optic core or lightdetector. The beam spot illumination may also land in various locationswithin the output region. An optical beam combiner may be included atthe output stage of the exemplary optical switch to cause all thepossible beam illumination spot locations to be superimposed, and tocorrect for the variation in the angles of incidence. Thus, with theinclusion of the beam combiner, each output light beam may be modifiedsuch that it can be coupled properly into an optical fiber or onto alight detector.

The principles of operation of an exemplary White cell on which thepresent optical photonic switch is based will be reviewed briefly inconnection with the illustration of FIG. 1. Referring to FIG. 1, theexemplary White cell 10 comprises three spherical mirrors B, C, and M.The mirror M faces the other two mirrors B and C, and is separated fromthem by a distance equal to their radii of curvature R, which is thesame for all three mirrors.

The center of curvature of mirror M (CC(M)) lies on the optical axis 12thereof. Because Mirrors B and C are mounted across from mirror M andseparated from it by a distance equal to the radius of curvature R,either mirror B or C images the surface of mirror M onto itself, whereasmirror M images B and C onto each other. The centers of curvature ofmirror B and C (CC(B) and CC(C), respectively) are located on mirror M,at a distance δ left and right of the optical axis 12, respectively.Hence the centers of curvatures CC(B) and CC(C) are separated by 2δ. Thelocations of the centers of curvature are key to the operation of abinary optical cross-connection device.

An exemplary path of a single light beam 16 through the White Cell 10 isshown by light rays in the top view illustrations of FIGS. 2 a, 2 b and2 c. FIG. 2 a shows how the light beam 16 enters the White Cell throughan input turning mirror 20 located adjacent to mirror M. The input lightbeam 16 is focused to a spot on the input turning mirror 20. Lightdiverging from this input spot will propagate toward mirror C and berefocused by mirror C back onto mirror M as illustrated in FIG. 2 a. Thespot illumination of the input light beam 16 on the input turning mirror20 is located at a distance d₁ away from the mirror C's center ofcurvature CC(C), and the first image of the spot illumination of lightbeam 16 on mirror M will therefore be located at point 22 an equaldistance d₁ from mirror C's center of curvature on the other side fromthe input turning mirror 20.

FIG. 2 b shows how the light beam 16 bounces from point 22 off mirror Mtowards mirror B. The light beam 16 diverges in its path towards mirrorB, but is refocused by mirror B onto mirror M as a spot image at point24. Since the first image at point 22 is located at a distance d2 fromone side of the mirror B's center of curvature CC(B), and then, thesecond image at point 24 will appear on mirror M at an equal distance d₂from the other side B's center of curvature.

A feature of the exemplary White cell 10 is shown in FIG. 2 c, where thelight beam 16 from mirror C is imaged via mirror M onto mirror B. Aslong as these two mirrors B and C are—the same size, light can be imagedback and forth between them many times without additional diffractionlosses from the edges of the mirrors. Therefore, the losses in thesystem are caused only by the mirrors' reflectivities.

This multiple-reflection White cell configuration 10 will result in anillumination spot pattern on the surface of mirror M. The spot patternas shown in the front view illustrations of mirror M in FIGS. 3 a, 3 b,and 3 c is very predictable depending only on the locations of thecenters of curvature of mirrors B and C. Each front view illustration ofFIGS. 3 a, 3 b and 3 c shows a sequence of spot illuminations on mirrorM for a particular input spot illumination on the input turning mirror20. The locations of the centers of curvature of each mirror B and C areindicated in each Figure. An output turning mirror 30 has been added tothe example to extract the light beam from the White cell 10 after allof the light beam reflections or bounces have been completed. Theilluminating spots in the FIGS. 3 a, 3 b and 3 c are numbered in theorder in which the light “bounces” in the White Cell before finallyimaging onto the output turning mirror 30. The odd-numbered spot imagesprogress across the top to the left of the mirror M and theeven-numbered spot images progress across the bottom to the right.

FIG. 3 a is an illustration for a single beam White cell operation asexemplified in the previous FIGS. 1 and 2 a-2 c. Referring to FIG. 3 a,the beam is directed to mirror C from the input turning mirror 20 andthere focused onto mirror M at spot image 1. From spot image 1 the beamis directed to mirror B and there focused into mirror M at spot image 2.From spot image 2, the beam is directed back to mirror C and therefocused onto mirror M at spot image 3. From spot image 3 the beam isdirected back to mirror B and there focused into mirror M at spot image4. The light beam will continue to bounce between mirrors B and C viamirror M for spot images 5 and above until the final bounce whichdirects the beam illumination or spot image to the output turning mirror30.

The spacing between the illuminating spot images for a given input beamis directly related to the distance 2δ between the centers of curvatureof mirrors B and C. The total number of spot images on mirror M istherefore dependent on the spacing δ and the overall size of mirror M.Note that the spot locations on mirror M depend entirely on thealignment of the two Mirrors B and C, and not on Mirror M. This willbecome of interest when we replace Mirror M with the MEMS micro mirrorsand the beam illuminating spot images are made to land on the tiltingmicro mirrors thereof.

A second beam may be introduced into the White cell 10 as shown in FIG.3 b rendering a simultaneous dual beam operation. In FIG. 3 b, one beamis represented by a square illuminating spot image and the other beam isrepresented by a triangle. Note that each input spot image from turningmirror 20 results in a different spot pattern on mirror M. In fact, asshown in FIG. 3 c, it is possible to introduce a large array of spotimages 40, each representing a different input signal. The spot imagepatterns on mirror M for each input beam are unique. In the presentexample, none of the bounces from any of the beams will strike any spotimage from another beam.

As noted above, Mirror M may be replaced with a MEMS micro mirror array,and two additional spherical mirrors may be added to form an alternateWhite cell 50 as shown in the illustration of FIG. 4. In this alternateWhite cell 50, each spot image from each beam introduced into the Whitecell 10 will strike a different micro mirror of the MEMS array. Thus, inthis alternate example, each beam in the array of input beams (see FIG.3 c, for example) may be independently controlled via the MEMS micromirrors on every beam bounce as will become more evident from thefollowing description. Thus, optical switching may be performed byallowing each input light beam to be switched between various White cellpaths that alter the spot patterns on mirror M and thus, the exitlocation of each beam. It is possible to allow for a very large numberof potential outputs for each of the input beams, but with the smallestpossible number of light beam bounces within the white cell. Reducingthe number of bounces reduces the loss, which will accumulate on everybounce.

Several cell configurations may be used to enhance the number ofpossible outputs with the least number of light beam bounces. The cellconfigurations may be divided in two categories: polynomial andexponential cells. In the “polynomial cells,” the number of possibleoutputs N is proportional to the number of bounces m raised to somepower. For example, in a quadratic cell N is proportional to m⁴, where mis the number of bounces on the MEMS device. In the “exponential cells,”the number of possible outputs is proportional to a base number raisedto the number of bounces (N is proportional to 2^(m) for the binarycase). The exponential approach has the advantage of providing far moreconnectivity for a given number of bounces (and thus loss), but thedisadvantage of not having the built-in redundancy of the polynomialdevices. In this application, all of these configurations will not bediscussed. A binary system will be briefly discussed to ease theintroduction of an optical beam combiner.

In the example of FIG. 4, an embodiment is illustrated which combinestwo White cells to produce an optical cross-connection device. Opticalswitching is performed by allowing each of a large number of input lightbeams to be switched between two different White cells. In thisembodiment, one White cell produces two rows of spot images for eachinput beam, and the second White cell incorporates a spot displacementdevice (SDD) that will continue the spot patterns but displace them bysome number of rows, thus changing the exit location of each beam. Avery large number of potential outputs are provided for each of theinput beams, but with the smallest possible number of bounces. Reducingthe number of bounces reduces the loss, which will accumulate on everybounce. In a “binary cell,” the number of possible outputs isproportional to 2^(m/4).

The architecture of the embodiment of FIG. 4 was originally proposed foroptical true time delay devices for phased array antennas. In theexemplary White cell described in connection with FIGS. 1-3 c, thelocation at which a spot image leaves the cell is determined by wherethe light beam entered the cell, and where the location of the centersof curvature of Mirrors B and C. In this alternate embodiment, the Whitecell is modified to control the output location of the spotillumination. To do this, Mirror M is replaced with a MEMS tiltingmicro-mirror array to select between two different paths on each lightbeam bounce. In addition, a second White cell is added in the newlyavailable path. Both White cells produce a similar spot pattern on theMEMS array, but the illuminating spot images resulting from the secondWhite cell are shifted such that they return in a different row of theMEMS array than if they returned from the first White cell.

Referring to FIG. 4 which illustrates an exemplary embodiment for abinary White cell device 50, mirror M is replaced with a MEMS micromirror array 52 and a field lens 54 disposed in front thereof. The MEMSarray/lens combination 52, 54 performs the imaging function of theoriginal spherical mirror M. On either side of the MEMS micro mirrorarray 52 may be disposed two flat auxiliary mirrors 56 and 58, whosefunctions will be described supra. Each of the auxiliary mirrors 56 and58 also has a field lens 60 and 62, respectively, disposed in frontthereof to simulate a spherical mirror. These three field lenses 54, 60and 62 may be combined into a single, larger lens as well.

The embodiment of FIG. 4 also includes four spherical mirrors 64, 66, 68and 70 disposed in front of the mirrors 52, 56 and 58, but instead ofhaving the centers of curvature of the spherical mirrors 64, 66, 68 and70 on the MEMS array 52, the centers of curvature are located by designoutside the MEMS array 52. In the present embodiment, the possible micromirror tip angles of the MEMS array 52 may be ±θ to the normal 72(dashed line) of the MEMS array 52. Mirrors 64 and 66 are disposed oneabove the other, along an axis 74 (dashed line) at an angle of −θ to thenormal axis 72. Mirrors 68 and 70 are also disposed one above the otheralong an axis 76 at an angle +3θ to the normal axis 72. While the mirrorsets 64, 66 and 68, 70 of the present embodiment are arranged one abovethe other, it is understood that the mirrors of each such set may bearranged side by side on either side of the respective −θ or +3θ axisjust as well. The axis of the lens 54 associated with the MEMS array 52is disposed along the normal axis 72; the center of curvature (labeledCC_(AI)) of the auxiliary mirror 56 and lens 60 together is disposed bydesign between mirrors 64 and 66, and similarly, the center of curvatureCC_(AII) of auxiliary mirror 58 and lens 62 is disposed by designbetween mirrors 68 and 70.

Let us assume that an input beam going from the plane of the MEMS array52 is directed to mirror 64, for example, after light beam bounce 1. Alight image reflected from this spot on mirror 64 is imaged to a newspot image on auxiliary mirror 56, in a column labeled “2” at the farleft thereof as shown in FIG. 4. From there, the light beam is reflectedto mirror 66, which directs the light beam back to the MEMS array 52 ata new micro mirror location, which may be in the column labeled “3”, forexample. If the micro mirror at that spot image of the MEMS array 52 isset to −θ, then the light beam is directed back to mirror 64 again. So,mirrors 64 and 66 form one White cell with the MEMS array 52, lens 54,auxiliary mirror 56, and lens 60.

Accordingly, when micro mirror of the MEMS array 52 that the light beamstrikes on bounce 3 is tipped to −θ, the light returns to auxiliarymirror 56 via mirror 64 and may be focused a spot in column 4, forexample. On the other hand, if the micro mirror of the MEMS array 52that the light beam strikes at bounce 3 is instead turned to +θ, thenthe light beam from mirror 66 will be reflected from the MEMS array 52at an angle of +3θ along the plane of axis 76 with respect to the normalaxis 72. Recall that there are two more mirrors 68 and 70 along the axis76. So, when the reflecting micro mirror is set at +θ, a light beam frommirror 66 will be directed to mirror 68 instead of mirror 64. In thepresent embodiment, a light beam is always directed to an upper mirror64 or 68 from the MEMS array 52.

When a light beam is directed from MEMS array 52 to mirror 68, the lightbeam is refocused to auxiliary mirror 58 and forms a spot image in acolumn 4 of that mirror, for example. From there the light beam isdirected to the lower mirror 70, and then back to the MEMS plane 52.Accordingly, mirrors 68 and 70 together with the MEMS array 52, lens 54,auxiliary mirror 58 and lens 62 comprise another White Cell of theembodiment. If the micro mirror in the MEMS array 52 struck by the lightbeam on bounce 5 is tilted to −θ, the light beam from mirror 70 is againdirected to the other White cell (specifically to mirror 64).Conversely, if the same micro mirror at bounce 5 is set tilted to +θ,the light beam from mirror 70 is instead reflected at +4θ, a directionthat is not being used in this design, and the beam is lost.

Thus, according to the connectivity diagram shown in FIG. 4 a, in thepresent embodiment, a light beam shown by the double arrowed line 80 maybounce continuously (and exclusively) between the MEMS array 52 andauxiliary mirror 56 via mirrors 64 and 66, a situation that doesn'toccur while bouncing through mirrors 68 and 70. A light beam directedfrom the mirror 66 to the MEMS array 52 may be directed either back tomirror 64 (see arrowed line 80) or to mirror 68 ( see arrowed line 82)depending on the reflection angle setting of the corresponding micromirror of the MEMS array 52. The light beam arriving at mirror 68 isreturned to the mirror 70 (see arrowed line 84) via auxiliary mirror 58.Then, from mirror 70, the light beam is directed back to the MEMS array52. Note that in the present embodiment, a light beam directed to theMEMS array 52 from mirror 70 must be directed to mirror 64 (see arrowedline 86) and auxiliary mirror 56; otherwise, it will be lost. Therefore,the light beam returning from the White cell comprising auxiliary mirror58 needs four bounces to be directed back to auxiliary mirror 58, i.e.one bounce from the mirror 58 to the MEMS array 52 via mirror 70, asecond bounce from the MEMS array 52 through mirror 64 to mirror 56, athird bounce from mirror 56 through mirror 66 to the MEMS array 52, anda fourth bounce from the MEMS array 52 to mirror 58 via mirror 68.

Note also that an input light beam may be sent to mirror 64 from theMEMS array 52 every even-numbered bounce, and to mirror 68 every fourthbounce (i.e. 4, 8, 12 . . . ). The odd-numbered bounces always appear onthe MEMS array 52, and the even-number spots can appear either onauxiliary mirror 56 or auxiliary mirror 58. The light beam may bedirected to auxiliary mirror 58 by the MEMS array 52 on any particulareven-numbered bounce, but when the light beam is directed there, fourconsecutive light beam bounces are required before the light beam may beresent to auxiliary mirror 58 again.

Now, suppose that in the embodiment of FIG. 4, the auxiliary mirror 58comprises a spot displacement device (SDD) that shifts a spot image overby some number of rows. This embodiment is exemplified in theillustrations of FIGS. 5 and 6. Referring to FIGS. 5 and 6, the SDD 58may be divided into columns, and each column is assigned to every fourthbounce. Also, the number of elements (pixels) or rows of each column ofthe array of the SDD 58 by which a beam is shifted will be different foreach column. That is, each column may shift a beam by a distance equalto twice that of the shift produced by the previous column. Thus, thefirst column will produce a shift of Δ, the second column a shift of 2Δ,the third column a shift of 4Δ and so on, then producing a binarysystem.

Shifting the spot images can control at which row any given input lightbeam reaches the output turning mirror and in the present example, eachrow may be associated with a different output. The number of possibleoutputs is determined by the total number of possible shifts for a givennumber of bounces. In the design of FIG. 6, a shift is made every timethe light beam is directed to the SDD 58, but this can only happen everyfour bounces. Thus the number of outputs N is given by:N_(binary)=2^(m/4)   (1)where m is the number of bounces.

In the mirror face diagrams of FIG. 5 is depicted a 12-bounce binaryWhite cell system to illustrate the operation of the embodiment of FIG.6. In this example, eight different beams, shown by various spot images,are incident on the input turning mirror 20. The patterns for the spotimages for three of the eight light beams are indicated in the faces ofthe mirrors 52, 56 and 58 which are each divided into a grid of eightrows (for eight possible output locations) and six columns (for eachbounce on the MEMS). The output column 90 constitutes a seventh columnnext to the MEMS array 52. In each region or pixel on the grid of theMEMS array 52 may be a group of eight micro mirrors, so that each of theeight beams may land on a different micro mirror on each bounce. Eachbeam may be directed either to the SDD 58 or to auxiliary mirror 56 oneach bounce. The number of columns on the SDD (m/4=3), will thusdetermine the number of possible outputs; the other columns 92 are notused. Every four bounces allows for a shift, so 12 bounces will produce2³=8 different outputs for each input light beam.

The example of FIG. 5 shows eight different input beams (only three,depicted by white, shaded and black symbols, being addressed in thepresent example) and eight possible outputs (numbered rows 0 to 7) inthe output column 90. Initially, the three input beams start on row zero(0). Remember that according to the connectivity diagram of FIG. 4 a, aninput light beam may only go to the 68, 70 White Cell every fourthbounce (those would be the 4th, 8th and 12th bounces for a 12 bouncesystem). In the present example, suppose that the “white” beam is to bedirected to the fifth output (row 5 of column 90), the shaded beam is tobe directed to the second output (row 2 of column 90), and the blackbeam is to be directed to row 0 of column 90. The spot images of thethree beams are shown in the respective mirror face for each bounce andthe bounce numbers are shown beneath the columns of the mirror faces.

In operation, the “white” beam should be directed to the SDD 58 on thefourth and twelfth bounces, which correspond to row displacements of 4Δand Δ, respectively. Accordingly, the “white” beam may initially bouncein the 64, 66 White Cell (i.e. the corresponding micro mirrors on theMEMS array 52 are tilted to −θ position) for three bounces. Then, the“white” beam is directed to the SDD 58 on the fourth bounce (i.e. thecorresponding micro mirror on the MEMS array 52 is tilted to +θ), andmore particularly to the column in the SDD 58 that has a shift value of4Δ. After being shifted four rows in the SDD 58, the “white” beam isdirected back to the MEMS array 52 on the fifth bounce and images on therow four (4) instead of row zero (0). The “white” beam is then keptbouncing in the 64,66 White cell, until the 12th bounce, when it isagain directed to the SDD 58, and more specifically directed to land inthe column with the shift value of Δ. After being shifted an additionalrow in the SDD 58, the “white” beam is directed back to the MEMS array52 on the next bounce and images on the row five (5) of the outputcolumn 90.

In a similar manner, the “shaded” beam may be shifted to the row two (2)of the output column 90 in twelve bounces (12). The “black” beam may beleft unshifted throughout the 12 bounces to be output at row zero (0) ofthe output column 90.

In the foregoing described embodiment, it is noted that any inputdirected to a particular output will land in a different place withinthat output region. For example, in FIG. 5, the white beam was directedto output five (row 5) and appeared as a spot image in the upper righthand corner. Had the black beam been sent to output five, its spot imagewould appear in the lower right hand corner. Thus, once a given inputhas reached the correct output region, the spot images should be allmade to land in the same spot, for example, for proper coupling to alight detector or a fiber core. This is non-trivial in the White cellbecause in addition to arriving at different locations in the outputregion, the light beams may arrive from different angles, a factor thatwill seriously affect output coupling, especially into an optical fiber.There are actually two angles of concern here. The first has to do withfrom which White cell of the two a beam is arriving when it reaches theoutput region. The other angle arises from the particular outputlocation within that region where the spot image forms.

The first angle is the more severe than the second. FIGS. 6 and 6 a showthe last bounce for two different beams 100 and 102 (i.e. the “white”and “shaded” beams, respectively, of FIG. 5) for the 12-bounce systemjust described. The “white” beam 100 is directed to the fifth output(row 5) of column 90, meaning it was shifted on its last bounce, so itis coming from the 68, 70 White cell. On the other hand, the “shaded”beam 102, directed to the second output (row 2) of column 90, comes fromthe 64, 66 White cell on its last bounce. Thus, the two beams 100 and102 are directed to their respective outputs from different White cells64, 66 and 68, 70.

One way to solve this condition of difference in which mirror the beamcomes from is to add one additional bounce to the system as shown in theillustration of FIG. 7. Then, regardless of the output row selected, allbeams may be directed back to the 64, 66 White cell on their lastbounce. The beams will come out at the appropriate row (i.e. output),and one column 104 over from column 90, but now all beams will arrive attheir respective output regions from the same general direction, that ofthe final spherical mirror 66. However, while the beams are all arrivingfrom the same White cell 64, 66, they are still directed to differentoutputs, i.e. rows of column 104 (FIG. 7). In addition, within eachoutput region, e.g. row 2 or row 5, each beam 100 and 102, for example,may arrive at any of several different locations (e.g. lower corner,middle) as shown in FIG. 6 a. This also creates a small difference inthe angle at which a beam arrives.

Furthermore, the light input to the multi-pass, cross-connection devicemay be a two-dimensional spot array, having both columns and rows.Therefore, all the rows and columns of the spot array should be combinedto a single spot, and this should be done taking into account thevarying angles of incidence. The output should be a single spot, ofsubstantially the same size and shape as any individual input spot, andthe output should emerge at a specific angle, independent of the arrivalangle of any particular beam. A method for superimposing all thepotential spot images onto a single location and with a single angleusing passive White cell technology will now be discussed.

An exemplary optical beam combiner 110 suitable for solving theaforementioned conditions is shown in the illustration of FIG. 8. In thepresent embodiment, passive (i.e. non-switching) White cell groups,which are examples of multi-pass spot generating optical systems, aredisposed at each of the three output regions, which will accept as itsinputs the spot arrays landing on each output region, i.e. row 0, row 2and row 5 (see FIG. 5). Referring to FIG. 8, the light beams 100, 102and 106 are shown arriving at their respective outputs from the opticalswitch 64, 66 (see FIG. 7). Included in the embodiment are threespherical mirrors 112, 114 and 116 which form multi-pass optical systemsor White cells with an analog mirror 118 to the MEMS mirror 52, whichmay be disposed on the backside of the MEMS mirror 52. Actually, theoptics of the present embodiment may be adjusted to place this analogmirror 118 in a more convenient spot, if desired. The light beams 100,102 and 106 may pass through their respective outputs and be incident onthe first 116 of three spherical mirrors 112, 114 and 116 as illustratedby the beam 120. While White cell groups are used in the presentembodiment, it is understood that other suitable multi-pass spotgenerating optical systems may be used as noted herein above withoutdeviating from the broad principles of the present invention.

The plane of mirror 118 comprises a passive flat mirror that has fixedtilted micro mirrors in some locations. These micro mirrors may beessentially small prisms whose hypotenuses are coated with a highreflectivity coating to direct a light beam incident at a particularpixel in a specific direction. This is in contrast to the MEMS device 52itself, which has micro mirrors at every location that may be tilted toa variety of directions. In the beam combiner 110, the angles of the“pixels” of mirror 118 may be fixed.

Suppose that in the present embodiment the output regions of the opticalswitch each contains a linear array of spot image positions such asexemplified in FIG. 9, for example. FIG. 9 is an illustration of theanalog mirror 118 that shows an input turning mirror 122 which is theinput to the beam combiner 110 and also the output of the opticalswitch. A physical input turning mirror 122 may not be needed, althoughfield lenses, not shown, may be. Each row 124 and 126 of the lineararray of positions shown in FIG. 9 corresponds to a different intendedoutput of the optical cross-connect device. In an opticalcross-connection device, more than likely, only one position of thepossible output spot image positions in each array 124 and 126 willactually be illuminated by an output light beam. Regardless of theposition in the array 124, 126 at which the beam arrives, it should bedirected to a single detector or optical fiber, corresponding to thatrow.

In the embodiment of FIG. 9, two different outputs of the optical switchwill be considered for an exemplary description of operation of the beamcombiner embodiment. One of the output beams is shown by a square symboland the other output beam is shown by a triangle symbol in FIG. 9. Inthe row 126 of the linear input array 122, the fourth position fromright to left is spot illuminated by the beam of the square symbol, andin the row 124, the second position from right to left is spotilluminated by the beam with the triangle symbol. Given this state asthe starting state, the linear array of light passing through the outputregion of the optical cross-connection device or optical switch, whichis the input 122 to the beam combiner 110, may be initially directed tomirror 116 as shown in FIG. 8. This spot array is imaged by mirror 116to a new spot array in the upper right hand corner of mirror 118 asshown in the FIG. 9.

A region 130 of the mirror 118 illuminated by the new spot arrayincludes a series of fixed micro mirrors, all tipped to some angle θ.The positions of the micro mirrors of region 130 correspond directly bycolumn and row to all of the spot image locations of the imaged spotarray. The tipped micro mirrors of region 130 direct the beams to mirror114 which, in turn, directs the beams back to mirror 118 to a region 132in the lower left hand corner thereof. Region 132 includes anotherseries of micro mirrors, all tipped to some angle. The positions of themicro mirrors of region 132 correspond directly by column and row to allof the spot image locations of the imaged spot array from mirror 114.

At this point the entire spot array image set has been stepped sidewaysby some distance greater than or equal to the original spot array size.The tipped mirrors of region 132 direct the entire beam array back tomirror 116, which, in turn, directs the beam array back to mirror 118 toilluminate another set of spot images in region 134 at the top leftcorner thereof. At region 134, there is another corresponding set ofmicro mirrors which are tipped to direct the entire spot array back tomirror 116, where another set of spot images are formed.

From here on in, each of the array imaged regions of mirror 118 mayinclude corresponding fixed micro mirror arrays that may be angled suchthat the light circulates only between mirror 112 and mirror 114. If itmay be arranged that a flat angle, e.g. the plane of mirror 118, may beall that is needed to circulate the light beam array between mirror 112and mirror 114, then no additional micro mirrors need to be added tomirror 118 at the array imaged regions thereof.

To achieve this result, the distance S′ between the centers of curvatureof mirrors 112 and 114 are set to be smaller than the centers ofcurvature between mirrors 114 and 116. Also, the sideways step describedherein above in connection with each bounce of the beam array will besmaller to the spacing between two spot positions in the linear array.In this design configuration, some of the spot images may land on arraypositions or pixels that have been previously visited by another spotimage of the array, but the direction of tilt of the micro-mirror is thesame so there is no adverse consequence. As the beams continue tobounce, each resulting spot illumination of a bounce will move one spotposition of the linear array over on each bounce. FIG. 9 shows thebounce numbers for each of the aforementioned two cases. After apredetermined number of bounces, the square symbol beam emerges from theWhite cell by falling through an exit port or “trap door” 136. In thepresent embodiment, the square symbol beam falls through the “trap door”136 on bounce number 7. The triangle symbol beam may take somewhatlonger to fall through a trap door 138, like on bounce number 11, forexample.

FIG. 10 provides a three-dimensional depiction of the embodiment of thebeam combiner embodiment of FIGS. 8 and 9 for a more detaileddescription of the operation thereof. In FIG. 10, the face of mirror 118is laid out on a grid to show the various spot illumination patterns ofthe beam array between bounces and the centers of curvatures of themirrors 112, 114 and 116 which are labeled as CC(A′), CC(B′) and CC(C′),respectively. Consider the fourth spot position 140 in the linear inputarray 122 of FIG. 9 which is depicted at approximately index 4 in thescale of FIG. 10. The light beam from this spot position 140 is directedfirst to mirror 116 or C′.

Since by design this mirror's center of curvature CC(C′) is located 12units from the input spot position 140 or approximately 16 on the indexscale, when the beam returns to mirror 118 from mirror 116, it isre-imaged at an approximate index location 4+2(12)=28 depicted by line142. At position 142, there is an angled or tipped mirror 146 whichdirects the light beam to mirror 114 or B′. Since by design the centerof curvature of mirror 114 or B′ is 10 units to the left of position142, the spot image from mirror 114 appears at an approximate location28−2(10)=8 depicted by a line 148. The angled or tipped mirror 150 atthis location 148 directs the light beam back to mirror 116 or C′,creating a return spot image on mirror 118 at an approximate indexlocation 23 depicted by line 152.

In region 152, the face of mirror 118 is flat. Thus, by design, thelight beam is directed from position 152 to mirror 114 or B′. Since thecenter of curvature of mirror B′ is set by design halfway between indexlocations 18 and 19, the spot image of the return beam from mirror 114will appear approximately at an index location 14 depicted by line 154.Therein after, the light beam may circulate by design only betweenmirrors 114 or B′ and 112 or A′. Accordingly, at the next bounce, thelight beam will illuminate a spot image at approximately an indexlocation 22 depicted by line 156, which may have already been visited onthe previous bounce by the fifth positioned beam in the linear inputarray, but it is of no consequence. Since the centers of curvature ofthe mirrors 112 and 114 are spaced one-half index unit apart, the spotimages of the light beam with each subsequent bounce will form one unitapart on each such bounce.

By bouncing exclusively between mirrors 112 and 114, any spot image of aparticular array will scan all the array positions ahead of it,eventually landing on each one. Suppose an exit port, like a hole or“trap door”, for example, is disposed at position 158 as shown in FIG.10 (see 136 and 138 in FIG. 9), then the first spot image in the arraywill fall through this hole at position 158 on its third bounce, andpass to an output fiber optic cable or a light detector, for example,that may be disposed behind it. In the same bounce, the other spotimages of the linear array, however, are still striking mirrors or themirror face, and continue bouncing in the White cell formed by mirrors112 and 114. At the fifth bounce, the second spot image of the arrayfalls through the hole 158; at the seventh bounce, the third spot imageof the array falls through the hole 158, and so on. While a hole or“trap door” is used for the exit port in the present embodiment, it isunderstood that other techniques, like another tipped mirror arranged todirect light out of the device, or a prism or grating cell arranged torefract or diffract light out of or from the device, for example, may beused just as well.

Note that in the foregoing described embodiment, the spot images of thelinear beam array all arrive at the same exit port or hole location,with the same angle of propagation, albeit at different times. Ifvariations in latency are a consideration, the light beams of the arraymay be pre-delayed in advance (in another White cell-based or otheroptical delay line, for example) such that when they pass through thebeam combiner 110, they exit at the same time as well. The tradeoff isadded complexity.

For a large cross-connection device or optical switch with many inputsand outputs, the spot images of the input beams may be in atwo-dimensional array. In this case, a second White cell group may beadded behind the first group 112, 114 and 116 to combine the rows ofeach region to a single spot. The optical losses of the beam combiner110 are expected to be very small, since all the optical elements arepassive, fixed, and may be treated with very high-reflectivity coatings.

In the operational example described in connection with the illustrationof FIG. 9 herein above, an input spot pattern of two rows were used byway of example. However, it is understood that by placing an input arraysuch that its spot images are colinear with the centers of curvatures ofthe mirrors 112, 114 and 116, the same operation may be performed, forone input spot array, using only one row, as described in connectionwith the illustration of FIG. 10. This simplification not only savesspace, but also allows an implementation of the beam combiner in aplanar waveguide.

FIGS. 11 and 12 illustrate an exemplary waveguide-based White cellembodiment of a beam combiner. Referring to FIGS. 11 and 12, theembodiment includes a planar waveguide 170, such that the light isguided in one dimension 172, which may be vertical direction, but actsas if it were in free space in the other direction, which may behorizontal to dimension 172. To implement a planar waveguide White cell,the implementation should include the waveguide equivalent of sphericalmirrors and the waveguide equivalent of a field lens. In the presentembodiment, three spherical mirrors may be implemented using threewaveguide lenses 174, 176 and 178 and a tilted flat mirror or mirrors180 behind them.

There exist different implementations of a lens for planar waveguidetechnology. For example, a geodesic lens, a chirped grating lens, or aLuneberg lens have all been documented in literature for several yearsas a suitable implementation of a waveguide lens. Any of these (orother) lens configurations may be used for the lenses 174, 176 and 178in a planar waveguide embodiment.

Still referring to FIGS. 11 and 12, light beams may enter the waveguide170 as spots at one end 182 thereof. Light beams from these spots areconfigured to travel in the direction 172 toward lens 178 or C″ and maydiverge in the horizontal direction to 172. The beam is passed through afield lens 184 disposed in its path to lens 178. The combination of thewaveguide lens 178 and the flat edge 180 of the waveguide back surface,which may be coated with a high-reflectivity coating, acts like a Whitecell objective mirror. The mirror surface 180 may be angled or “tipped,”to properly locate the center of curvature of the effective mirror178/180 or C″.

From mirror 178/180, the light beam is re-imaged at the input edge 186of the waveguide beam combiner 170. At the first image location 188,there may be a series of angled or tipped micro mirrors 190, which couldbe etched into the waveguide input edge 186 or be micro prisms that areglued to the input face 186, for example. In any case, the micro mirrors190 are also coated for high reflectivity. The tip angle of the micromirrors 190 may be such that they send the beams to mirror 176/180 orB″.

The input edge or face 186 of the waveguide beam combiner 170 isillustrated in FIG. 11, which includes two sets of micro-prisms 190, 192and 194, 196, for example. The rest of the input edge 186 may be leftflat, but coated everywhere except for an exit port 198, which may be ahole, a gap, or “trap door”, for example, which is left un-coated (orAR-coated), such that the light beam may pass through and out of thecombiner 170. Other possible exit ports which have been noted hereinabove may be used just as well. As with the free space embodimentdescribed supra, every spot image of the light beams exits the combiner170 at the same point and with the same angle, but at different times.

In summary, an apparatus and method are described for combining lightbeams coupled from an optical cross-connection device at differentspatial locations and different angles. Also, such light beams arecombined to a single spot with a single arrival angle. While light beamsoutput from a White-cell based optical cross-connection device wereutilized herein above to describe various embodiments of the beamcombiner by way of example, it will be appreciated that the beamcombiner could be applied to other situations in which beams need to besuperposed. The superposition is achieved in the exemplary embodimentsby introducing all the beams into a White cell, and using the White cellto shift each beam over by one position or slot on each bounce or pass,until the light beam falls or passes through an exit port leading out toanother optical device. At the exit port, the spot images may be allsuperimposed in space (but not in time), and have the same angle ofpropagation as the corresponding light beams are all coming from thesame direction.

A binary White cell optical cross-connection device was used by way ofexample in the above descriptions for the purposes of discussion, butthe beam combiner solutions apply equally well to any optical devicewhich combines multiple beams into a single beam, a multi-pass opticalcross-connection device being one example. Further, the beam combinermay be applied anywhere were beams arriving from different places andfrom different angles should be superimposed. A three-dimensional Whitecell beam combiner arrangement with spherical mirrors and lenses may beused to combine an array of rows of light beams to a single column ofspot images, albeit at different times. In addition, the light beams ofthe array may be pre-delayed in advance (in another White cell-based orother optical delay line, for example) such that when they pass throughthe beam combiner 110, they may exit at the same time as well.Alternatively, the beam combiner may be embodied in a waveguide approachin which one waveguide for each row of light beams may be used tocombine all of the beams.

While the present embodiment has been described herein above inconnection with a plurality of embodiments, it is understood that suchpresentations were made merely by way of example with no intent oflimiting the invention to any single embodiment or a combination ofembodiments. Rather, the present invention should be construed inbreadth and broad scope in accordance with the recitation of the claimsappended hereto.

1. An optical beam combiner for combining a plurality of light beams,said beam combiner comprising: a plurality of spherical mirrors; and aflat mirror, said plurality of spherical mirrors and said flat mirrorconfigured to form at least one multiple pass light beam opticalarrangement for receiving said plurality of light beams and forsuperimposing spot images of said light beams onto a single locationwith a single incident angle.
 2. The optical beam combiner of claim 1wherein the single location is at an output of the beam combiner.
 3. Theoptical beam combiner of claim 1 wherein the flat mirror includes anexit port; and wherein the single location is at said exit port of theflat mirror.
 4. The optical beam combiner of claim 1 wherein the spotimages of each of the plurality of light beams are superimposed onto thesingle location at different times.
 5. The optical beam combiner ofclaim 1 wherein all of the plurality of spherical mirrors are configuredto have their centers of curvature on the surface of the flat mirror,said centers of curvature of the plurality of spherical mirrors beingspaced apart predetermined distances in relation to each other.
 6. Theoptical beam combiner of claim 1 wherein the flat mirror comprises atleast one fixed angle micro mirror disposed at each of a plurality ofpredetermined locations at the flat mirror.
 7. The optical beam combinerof claim 6 wherein the micro mirrors are disposed on a surface of theflat mirror.
 8. The optical beam combiner of claim 6 wherein each of themicro mirrors comprises a micro prism.
 9. The optical beam combiner ofclaim 6 wherein the predetermined locations include locations on asurface of the flat mirror which are illuminated by the plurality oflight beams.
 10. The optical beam combiner of claim 1 wherein theplurality of spherical mirrors comprises three spherical mirrors, allthree spherical mirrors having their centers of curvature on the surfaceof the flat mirror, said centers of curvature of the three sphericalmirrors being spaced apart predetermined distances in relation to eachother.
 11. An optical beam combiner for combining an array of lightbeams, said beam combiner comprising: a plurality of spherical mirrors;and a flat mirror, said plurality of spherical mirrors and said flatmirror configured to form at least one multiple pass light beam opticalarrangement for receiving simultaneously said array of light beams andfor superimposing spot images of each light beam of said array onto asingle location with a single incident angle.
 12. The optical beamcombiner of claim 11 wherein the single location is at an output of thebeam combiner.
 13. The optical beam combiner of claim 11 wherein theflat mirror includes an exit port; and wherein the single location is atsaid exit port of the flat mirror.
 14. The optical beam combiner ofclaim 11 wherein the spot images of the array of light beams aresuperimposed onto the single location at different times.
 15. Theoptical beam combiner of claim 11 wherein the flat mirror comprises anarray of fixed angle micro mirrors disposed at each of a plurality ofpredetermined locations on a surface of the flat mirror; and whereinsaid arrays of micro mirrors are configured to bounce the array of lightbeams to pre-designated spherical mirrors of the plurality within themultiple pass light beam optical arrangement.
 16. The optical beamcombiner of claim 15 wherein the light beams of the array bouncesimultaneously among the plurality of spherical mirrors and flat mirrorwithin the multiple pass light beam optical arrangement.
 17. The opticalbeam combiner of claim 16 wherein the single location is at the surfaceof the flat mirror; wherein the spot images illuminated by the array oflight beams on the surface of the flat mirror shift along the surface atcertain bounces toward said single location to be positioned thereat;and wherein the light beams of the array continue to bounce among theplurality of spherical mirrors and flat mirror within the multiple passlight beam optical arrangement until all of said spot images of thearray of light beams are superimposed at said single location.
 18. Awaveguide-based optical White cell comprising: a waveguide having frontand rear edges, the inside surfaces thereof being coated with areflective material, wherein said front edge including an input sectionfor the passage of at least one light beam into said waveguide; at leastone waveguide lens disposed in front of the inside surface of said rearedge to form a plurality of waveguide spherical mirrors at said rearedge; a plurality of angled micro mirrors disposed at the inside surfaceof said front edge; and said plurality of waveguide spherical mirrorsand said coated front edge configured to form at least one waveguideWhite cell.
 19. The waveguide-based optical White cell of claim 18wherein the White cell is configured for receiving a plurality of lightbeams and for superimposing spot images of said light beams onto asingle location at the front edge with a single incident angle.
 20. Thewaveguide-based optical White cell of claim 19 wherein the front edgeincludes an exit port; and wherein the single location is disposed atsaid exit port at the front edge.
 21. The waveguide-based optical Whitecell of claim 18 wherein the rear edge of the waveguide is angled tolocate the centers of curvature of the plurality of waveguide sphericalmirrors at the front edge of the waveguide, said centers of curvature ofthe waveguide spherical mirrors being spaced predetermined distancesfrom each other at the front edge to form the waveguide White cell.